Immunofluorescence tests on platelets have always been hampered by nonspecific fluorescence caused by non-immunological binding of plasma proteins to the platelet membrane. It was found that this could be easily overcome by fixation of the cells with paraformaldehyde (PFA). By using PFA-fixed platelets, a simple method for the detection of platelet antibodies, the platelet suspension immunofluorescence test (PSIFT) was developed. PFA fixation did not alter or inactivate the platelet antigens tested. Platelet-reactive antibodies detected specifically with the PSIFT included platelet-specific agglutinins of the IgM class, non-agglutinating platelet-specific antibodies of the IgG class, drug-dependent platelet antibodies, HLA antibodies, as well as anti-A and anti-B antibodies. The sensitivity of the new test was satisfactory, as was its reproducibility. Measurement of platelet immunofluorescence was possible in a continuous flow microfluorometer, making a principle, quantitation of platelet antibodies and antigens possible.

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After a survey of the literature dealing with the demonstration of platelet autoantibodies by immunofluorescence techniques, the results are given of a study in which immunofluorescence microphotometry was used for this purpose. The serum of 58, the platelets of 34, and the megakaryocytes of 2 patients with thrombocytopenia were investigated. In 21 of 52 sera (40%) in which the presence of platelet autoantibodies could be expected, positive results were obtained that could not be due to isoantibodies, either because the patients had not been pregnant and had not received blood transfusions or because the reactivity of the serum with the patient's own platelets was demonstrated. The platelets of 28 patients with thrombocytopenia not due to a platelet defect or decreased thrombopoiesis were investigated. In the platelets of 15 (54%) of these, significant differences in fluorescence were found with anti-immunoglobulin conjugate as well as with anti-IgG, -IgA, -IgM, or -complement reagents. It was concluded that in these patients in vivo sensitization of the platelets with autoantibody was demonstrated. In two patients an indication of the in vivo sensitization of the megakaryocytes was also obtained.

The expression of HLA-A and -B antigens on peripheral blood lymphocytes and blood platelets was measured using monoclonal antibodies in a semi-quantitative ELISA technique. Reactively of monoclonal anti-HLA-A2 and anti-HLA-B7, with lymphocytes as well as platelets, was in agreement with the presence of these antigens as detected by conventional HLA typing of lymphocytes. When the actual amount of HLA antigens was measured, a gene-dose effect was seen: cells from HLA-B7-homozygous individuals bound more monoclonal anti-HLA-B7 antibodies compared to their HLA-B7-heterozygous siblings. At the same time, cells of different donors showed only very small differences in binding of monoclonal antibody against an HLA-"backbone" determinant. Relative to total HLA-A, -B and -C expression, the amounts of HLA-A2 on lymphocytes and platelets were similar. On the other hand, the expression of HLA-B7 on platelets was diminished compared to that on peripheral blood lymphocytes.